Occasional darkening of the eyes is a transient blackout. In young people who have been working late at night or after a long period of eye fatigue and suddenly experience a transient loss of vision that can be recovered with a few moments of rest, it is considered to be a transient darkening in front of the eyes due to visual fatigue or a spasm of the blood vessels in the fundus of the eye. It is necessary to rest immediately and stop using the eyes to prevent blockage of the central retinal artery. There are many causes of this symptom, usually related to fundus hypoperfusion, which is a precursor of fundus vascular obstruction, including central retinal artery obstruction, and central retinal vein obstruction, mostly in the elderly, who have hypertension, hyperlipidemia, atherosclerosis or thick blood. Glaucoma patients with intermittent IOP elevation and optic papillary edema compressing the fundus vessels can also develop transient vision loss, blackness and darkness in front of the eyes, and the symptoms are reversible after IOP recovery. If you have frequent recurrence of transient darkness, you need to go to the hospital promptly.